Coastal Single-beam Bathymetry Data Collected in 2022 From Breton Island, Louisiana

GNSS Acquisition: Two Global Navigational Satellite Systems (GNSS) base stations were established on NGS benchmarks in which the roving vessels would be within 10 kilometers (km) of the base station at a given time. The primary benchmark BRET was approximately located in the middle of the survey extent and the secondary base station BRT2 was located to the northeast. The base stations were continually occupied and equipped with a Spectra Precision SP90M GNSS receiver recording full-carrier-phase positioning signals from satellites via Trimble Zephyr 3 Base GNSS antennas recording at a rate of 0.1 second (s).

Single-Beam Bathymetry Acquisition: The single-beam bathymetric data collected under the USGS FAN 2022-328-FA, include three separate survey platforms/subFANs; the R/V Twin Vee (subFAN 22BIM07), the R/V Shark (WVR1 subFAN 22BIM08) and the R/V Chum (WVR2 - subFAN 22BIM09), the 12-foot Yamaha PWCs. The R/V Twin Vee collected 253.85 line-km, the R/V Shark collected 138.53 line-km, and the R/V Chum collected 163.00 line-km. Boat motion was recorded on each vessel at 50-millisecond (ms) using a SBG Ellipse A motion sensor. HYPACK A Xylem Brand, a marine surveying, positioning, and navigation software package, managed the planned-transect information and provided real-time navigation, steering, correction, data quality, and instrumentation-status to the boat operator. Depth soundings were recorded at 50-ms intervals using an Odom echotrac CV100 echosounder with a 4-degree, 200-kilohertz (kHz) transducer. For each vessel, data from the GNSS receiver, motion sensor, and echosounder were recorded in real-time and merged into a single raw data file (.RAW) in HYPACK, with each device string referenced by a device identification code and time stamped to Coordinated Universal Time (UTC). Sound velocity profile (SVP) measurements were collected using SonTek Castaway Conductivity, Temperature, and Depth (CTD) instruments. The instruments were periodically cast overboard to record changes in water column speed of sound (SOS). A total of 87 successful sound velocity casts were collected and ranged in depth from 0.15 to 12.65 m, and in sound velocity from 1515.17 to 1542.43 meters per second (m/s).

Differentially Corrected Navigation Processing: The USGS installed a benchmark on Breton Island in 2007, stamping BRET, but does not have an associated Permanent Identifier (PID) as it is not in the NGS database. The coordinate values of the GPS base station (BRET) are the time-weighted average of values obtained from the NGS OPUS. The GPS and/or GNSS (based upon antenna type) occupations occurred during surveys in 2007, 2014, and 2022. The base position used for post-processing the navigation in this data release was 29°29’38.33211 North (WGS84-G1762), 89°10’29.28099 West (WGS84-G1762), and -25.254 meters (m) ellipsoid height (WGS84-G1762). The kinematic trajectories (rover to base) and base station coordinates were imported and processed using NovAtel’s Waypoint GrafNav software. Each kinematic GNSS data session from the survey vessel was post-processed to the concurrent base GNSS data session. Analyzing the data plots, trajectory maps, processing logs, satellite health plots, and other viewing utilities that GrafNav produces, provided measures to attain trajectory solutions (between the base and the rover) free of erroneous data that resulted in fixed positions. Some examples include 1) excluding satellites flagged by the program as having bad/poor health or cycle slips, 2) Excluding poor satellite time segments that have a negative influence toward a fixed solution, and 3) adjusting the satellite elevation mask angle to improve the position solutions (DeWitt and others, 2016; Hansen and others, 2017). The final differentially corrected, precise DGPS positions were computed at 0.1 s and exported in American Standard Code for Information Interchange (ASCII) text format in WGS84 (G1762) UTM 16N geodetic datum.

Bathymetry Processing: All data were processed using CARIS HIPS and SIPS. First, a vessel file *.vhf unique to the platform, was created that contains the surveyed offsets measurements between the sensors. A CARIS project was created using the *.vhf file, and then the HYPACK .RAW and the SPV profiles *.SVP were loaded. Next, the differentially corrected navigation files were imported using the generic data parser tool. Then, the differently corrected navigation files overwrote every non-differential position in the HYPACK .RAW file based upon time. The bathymetric data components (position, motion, depth, and SOS) were then georeferenced and geometrically corrected in CARIS to produce processed x,y,z positional data. Once georeferenced, the data were reviewed and edited for outliers, including further review in the Subset Editor utility for crossing status, and questionable areas or data points were removed. The geometrically corrected point data were then exported as an x,y,z ASCII text file referenced to WGS84 (G1762) UTM 16N (DeWitt and others, 2016; Hansen and others, 2017).

Quality Control, Quality Assurance (QA/QC) and Uncertainty Analysis: The single-beam data (all vessels contained in one file) exported from CARIS HIPS and SIPS (x,y,z ASCII text file) were transformed in Esri ArcMap to a point shapefile (.shp) utilizing the "Create Feature Class From XY Table" geoprocessing tool. The projection was set to WGS84 UTM 16N. The generated shapefiles were visually reviewed for any obvious outliers or problems. Next, polyline shapefiles (representing tracklines) was produced from the point shapefiles using XTools Pro "Make Polylines from Points" geoprocessing tool for each survey platform (subFANs 22BIM07, 22BIM08 and 22BIM09). Utilizing both the x,y,z (point) and trackline (polyline) shapefiles, a Python script was used to evaluate elevation differences at the intersection of crossing tracklines by calculating the elevation difference between points at each intersection using an inverse distance weighting equation with a search radius of 5 m. The crossing analysis yielded a 6.30 cm root-mean-square (RMS) error for all crossings (DeWitt and others, 2016; Hansen and others, 2017). When the R/V Twin Vee crossed one of its own lines, the crossing analysis yielded a 2.23 cm RMS error. When the R/V Twin Vee crossed a line surveyed by the R/V Chum, the crossing analysis yielded a 12.36 cm RMS error. When the R/V Twin Vee crossed a line surveyed by the R/V Shark, the crossing analysis yielded an 8.03 cm RMS error. When the R/V Chum crossed one of its own lines, the crossing analysis yielded a 6.48 cm RMS error. When the R/V Chum crossed a line surveyed by the R/V Shark, the crossing analysis yielded a 7.11 cm RMS error. When the R/V Shark crossed one of its own lines, the crossing analysis yielded a 6.48 cm RMS error. The crossings in ArcMap were solely used as a QA/QC and the ASCII export from CARIS was used as the final data product. The ASCII text file can be downloaded from this data release in Breton_Island_2022_SBES_WGS84_UTM16N_xyz.zip, and the trackline shapefile from Breton_Island_2022_SBES_WGS84_UTM16N_tracklines.zip.

Datum Transformation: NOAA/NGS's VDatum was used to transform the single-beam data points' horizontal and vertical datums (x,y,z). The first transformation was from WGS84 (G1762) UTM 16N into NAD83 (horizontal) UTM 16N and NAVD88 (vertical) using GEOID18 with a reported vertical uncertainty of 0.0671095 m. VDatum reports a nationwide standard deviation of 2.0 centimeters (cm) for ellipsoid - NAD83 transformations, and a nationwide standard deviation of 5.0 cm for NAD83 to NAVD88 transformations in the coastal regions of the continental U.S. For more information about the positional accuracy for these datum transformations, visit the Estimation of Vertical Uncertainties VDatum webpage, https://vdatum.noaa.gov/docs/est_uncertainties.html. The resultant ASCII text data files can be downloaded from this data release in Breton_Island_2022_SBES_NAD83_NAVD88_UTM16N_GEOID18_xyz.zip.